The present invention relates to a suspension control system for use in vehicles.
One example of conventional suspension control systems is disclosed in Japanese Patent Application Unexamined Publication Number (hereinafter referred to as "JP(A)") 5-330325 (U.S. Pat. No. 5,533,597). According to the prior art, at the beginning of going up an undulation of the road surface (i.e. the surface of a paved road that has a series of gentle undulations), the vehicle body is displaced upwardly, and the shock absorber is compressed. The system has been arranged such that the damping coefficient at this time is set to a value that provides "hard" damping force characteristics for the extension stroke and "soft" damping force characteristics for the compression stroke (hereinafter referred to as "extension hard/compression soft characteristics"). Therefore, the road surface change is unlikely to be transmitted to the vehicle body, and a favorable ride quality is obtained. However, on the way to the top of the undulation, the shock absorber begins to extend by the action of the spring compressed when the vehicle began to go up the undulation, causing the vehicle body to move upwardly at a relatively high velocity in cooperation with the inertia acting on the vehicle body as the vehicle goes up the undulation. Even after the vehicle has passed the top of the undulation, the shock absorber remains in the extension hard/compression soft characteristic conditions and is therefore difficult to extend. Accordingly, the vehicle body is pulled downwardly by the unsprung load, and the downward acceleration acting on the vehicle body increases. Consequently, the occupant of the vehicle may feel uncomfortable, having a sensation of being thrown out upwardly.
At the beginning of going down an undulation of the road surface, the vehicle body is displaced downwardly, and the shock absorber is extended. The system has been arranged such that the damping coefficient at this time is set to a value that provides "soft" damping force characteristics for the extension stroke and "hard" damping force characteristics for the compression stroke (hereinafter referred to as "extension soft/compression hard characteristics"). Thereafter, on the way to the bottom of the undulation, the shock absorber is compressed by the inertia acting on the vehicle body as the vehicle goes down the undulation. At this time, since the damping coefficient assumes a value for the compression hard characteristics, the upward acceleration rapidly increases. Consequently, the occupant may feel uncomfortable, having a sensation of being strongly pressed against the seat.
To solve the above-described problems, the present applicant proposed a suspension control system as disclosed in JP(A) 7-304315 (U.S. Ser. No. 405,873). The suspension control system has a variable-damping coefficient shock absorber interposed between a sprung member and unsprung member of a vehicle; an actuator for adjusting the damping coefficient of the variable-damping coefficient shock absorber; a damping force control device that sends a control signal to the actuator to adjust the damping coefficient according to the running condition of the vehicle; a vertical acceleration detecting device for detecting a vertical acceleration acting on the vehicle body; and a control signal adjusting device that adjusts the control signal sent to the actuator such that the damping coefficient of the shock absorber is reduced when the vertical acceleration exceeds a preset vertical acceleration reference value. With this arrangement, when the vertical acceleration exceeds the reference value on such an occasion that the vehicle has reached the top or bottom of an undulation of the road surface, the control signal sent to the actuator is adjusted such that the damping coefficient of the shock absorber is reduced, thereby preventing the vertical acceleration from being undesirably enhanced by the damping force generated by the shock absorber, and thus preventing the occupant from feeling a sensation of being thrown out upwardly or strongly pressed against the seat and enabling the occupant to enjoy a comfortable ride.
However, the above-described conventional suspension control system (JP(A)7-304315) suffers from the following problem: In the prior art, a signal representative, for example, of a vertical acceleration acting on a sprung member at a position corresponding to a variable-damping coefficient shock absorber provided for each wheel is detected, and the damping coefficient of each shock absorber is controlled on the basis of the detected vertical acceleration signal. Accordingly, control is effected for each wheel after a vertical acceleration or the like has occurred on the sprung member. Therefore, satisfactory control effect cannot be obtained. Especially, at the rear end of the vehicle, where shock absorbers are likely to be disposed near the occupant, he or she may feel a sensation of being thrown out upwardly or strongly pressed against the seat when the vehicle has reached the top or bottom of an undulation.